This invention relates to an improved method of concentrating by multiple effect evaporation solutions containing mixtures of salts and particularly this invention relates to concentrating by multiple effect evaporation solutions containing a first and a second salt, where the solubility of the first salt increases more with increasing temperatures than the solubility of the second salt. Even more particularly, this invention relates to an improved method whereby the solution concentrated contains principally potassium chloride, sodium chloride and a minor amount of salt impurities such as chlorides and sulfates of magnesium and calcium.
Salts having a solubility that increases with increasing temperatures within a given temperature range, hereinafter called first salts, and salts having a solubility that remains relatively unchanged or decreases with increasing temperatures within the same temperature range, hereinafter called second salts, are frequently encountered as mixtures in naturally occuring ores. In recovering such salts, the ore is dissolved in a suitable aqueous solvent forming a solution from which the salts can be easily separated. Admixtures of these salts in solution can also arise as a result of industrial chemical production, e.g., as a result of the electrolysis of electrolytes.
These salts can be separated from the solution by concentrating the solution by evaporation to produce a solution in which the first and second salt are concentrated to their "invariant composition". By "invariant composition" is meant a composition in which a solution at a given temperature is saturated with respect to two or more salts. This solution is forwarded to a recovery zone where the first salt is recovered, e.g., by cooling the solution so that the first salt will selectively crystallize out of solution and precipitate. The temperature range at which the water removal step and cooling step takes place is a range in which the first salt and second salt maintain their solubility characteristics. If the second salt is initially in high enough concentration, it will be precipitated and can be recovered during the initial evaporation step. Otherwise, subsequent evaporation after recovering the first salt can yield production of the second salt. Thus, alternate evaporation and cooling can substantially deplete the solution of the first and second salt.
Potassium chloride (first salt) and sodium chloride (second salt) are recovered commercially from naturally occurring ores comprising principally potassium chloride and sodium chloride and to a lesser extent a minor amount of salt impurities such as chlorides and sulfates of magnesium and calcium, e.g., less than 6 percent of salt impurities. In this commercially practical process, water is removed from the solution by evaporation until the solution approaches or reaches its invariant composition. Large amounts of sodium chloride and some salt impurities are precipitated and sodium chloride removed during the evaporation step. The solution is then purged of impurities precipitated during evaporation and cooled to crystallize potassium chloride while other salts and impurities remain in solution.
The invariant composition of potassium chloride-sodium chloride solutions is affected by other salts in the solution. For example, solutions of many naturally occurring potassium chloride-sodium chloride containing ores also comprise chlorides, carbonates, sulfates and the like of anions other than sodium and potassium, as hereinbefore mentioned. The presence of some of these other salts will lower the salt concentration of the invariant composition from the concentration found for a mixture of only sodium chloride and potassium chloride. For example, the presence of a few parts magnesium chloride per hundred parts water will lower the invariant composition by a few parts each of sodium chloride and potassium chloride.
Evaporation of potassium chloride-sodium chloride solutions are carried out with great expediency by backward fed multiple effect evaporation to achieve high product recovery and great steam economy. That is, mother liquor effluent overflow from cooler evaporator effects is forwarded to hotter evaporator effects. To obtain a satisfactory working temperature difference between the first (hottest) evaporator effect and the last (coolest) evaporator effect, the first evaporator effect is operated under super atmospheric pressure and the last evaporator effect is operated under vacuum. As the solution passes through each evaporator effect, water is removed in the form of vapor and the solution becomes concentrated with respect to potassium chloride while precipitating sodium chloride which settles into and is removed from an elutriation leg in communication with the bottom of each evaporator. Sodium chloride will precipitate until the solution reaches its invariant composition for the temperature at which each evaporator effect is operated. Impurities which have solubility characteristics of second salts may be precipitated as well during the process but are fluidized by an elutriating liquid so that sodium chloride can be recovered relatively pure.
Since the feed solution is relatively cool, the evaporator effects are heated by steam in a direction opposite to that of the solution, i.e., backward feed. The first evaporator effect is heated by introducing steam from an external source, such as a boiler, and the second evaporator effect is heated with vapors from the first evaporator effect and so on, progressively to the last evaporator effect whose vapor is utilized for whatever requirement low value steam can serve or is condensed and cooled for use as a coolant.
Mother liquor effluent overflow from the first evaporator effect is transferred to a solids settling zone or thickener. In this zone, fine particles of salt impurities which were precipitated in each evaporator effect and carried forward with mother liquor overflow are allowed to settle. Typically, the settling zone is operated at atmospheric pressure and under quiescent conditions in order for the settling to take place to facilitate removal of the solids from the mother liquor. Clarified mother liquor therefrom can then be forwarded to the step in which potassium chloride is recovered. So, it is a desideratum that mother liquor from the evaporation is not at super atmospheric pressure, the reduction of which to atmospheric pressure causes flashing. Flashing results in agitation of the mother liquor in the settling zone, thereby making it difficult for the settling of the fine particles to take place. Also, flashing produces the undesirable result of cooling the mother liquor which is as hereinbefore described at its invariant composition, the result of which is precipitation and loss of potassium chloride along with fine particles of impurities already in the solution.